The document DE 1 174 113 B1 discloses a generic axial cage for an axial needle roller bearing that consists substantially of a thin-walled annular disc having a W-shaped profile cross-section, the individual profile limbs of which are formed by a radially inner rim that is integrally formed on the inner edge of the annular disc, by a radially outer rim that is integrally formed on the outer edge of the annular disc and by a U-shaped bead which is connected via straight profile sections to the rims and has the same radial extension as the rims. A plurality of rectangular cage pockets arranged at uniform distances apart from each other is recessed out of the bead, between said cage pockets are formed pocket webs that connect the rims to each other through which pocket webs, the cylindrical rolling elements configured as bearing needle rollers are retained in the cage pockets and guided in circumferential direction.
However, it is sufficiently known from practice that a so-called needle boring caused by centrifugal forces acting on the bearing needle rollers occurs in such axial cages during bearing operation. Due to this needle boring, the bearing needle rollers come with their outer end faces into frictional contact with the corner radii of their cage pockets. Due to this frictional contact, the corner radii, provided at these points with a view to enhancing the shearing strength and the shape stability of the axial cage, work or wear out little by little till, finally, the axial cage breaks or fails prematurely.
For avoiding such a premature failure of the axial cage caused by needle boring, DE 101 43 089 A1 therefore proposes to configure the inner and/or outer rim of the axial cage in such a way that the rim comprises a circumferential vaulted portion in the transition region to the straight profile sections and merges with a circumferential constriction in the region of its free profile end. This circumferential constriction protrudes into the cage pockets and is in supporting contact with the end faces of the bearing needle rollers at two points of contact such that the bearing needle rollers can no longer run against the corner radii of the cage pockets under the influence of centrifugal forces.
In this solution which in itself is very advantageous, it has been determined, however, that it can be realised from the production point of view only with additional expense or with a completely modified method technology. The usual method technology for manufacturing an axial cage without chip removal consists, as known, of the steps of making the axial cage out of an endless sheet metal strip in a plurality of work steps using a stepped punching and drawing tool in which, at first, the rims are worked into the sheet metal strip, followed by punching out the cage pockets and the inner diameter of the axial cage, then forming the bead integrally between the rims and finally also punching out the outer diameter of the axial cage. At the end of the individual work steps, the completed axial cage is still connected to the sheet metal strip merely through two lateral retaining webs formed on its outer rim, from which the axial cage is separated in a last work step. However, if, as is the case in the described solution, this rim comprises a circumferential constriction that merges into a circumferential vaulted region, it is not possible, due to the vaulted region arranged under the retaining webs, to separate the completed axial cage from the lateral retaining webs directly at the rim because, otherwise, a damaging of the vaulted region has to be expected. Separation of the axial cage from the retaining webs must therefore be performed at the inner rim of the sheet metal strip, so that the retaining webs are at first still present on the separated axial cage and must be removed separately at a high cost. Another method of manufacturing the described axial cage would be to make it out of separate sheet metal circular blanks. This method, however, necessitates complex and expensive transportation devices that transport the circular sheet metal blanks further from one work step to the other, so that for this very reason the costs of this manufacturing method are disadvantageously raised.
Another possibility of avoiding the described disadvantageous needle boring in an axial cage has been additionally disclosed in EP 2 103 825 A1. This document proposes forming, within the cage pockets on the straight profile sections arranged between the rims of the axial cage, two wedge-shaped sheet metal lugs on each rim side. When centrifugal forces act on the bearing needle rollers, said wedge-shaped lugs come into supporting point contact with the end faces of the bearing needle rollers at the level of their central longitudinal axes.
A solution of the aforesaid type has, however, also proved to be disadvantageous because the wedge-shaped sheet metal lugs are arranged on the straight profile sections and these straight profile sections usually form the deepest plane of the axial cage. In order to be able to raise the straight profile sections structurally to the level of the central longitudinal axes, the rims of the axial cage must comprise a radial extension in opposite direction to the cage webs which get formed by reason of the worked-in bead. This, however, again has a detrimental effect on the overall stability of the axial cage and likewise additionally creates technical manufacturing problems because, after separation from the sheet metal strips, such axial cages are usually collected in collecting boxes for transportation to the final finishing station. By reason of the pocket webs which protrude freely from the straight profile sections in opposite direction to the rims, the danger arises that the axial cages get entangled with one another in the collecting boxes through their pocket webs. This can therefore only be avoided with the provision of additional stacking devices that likewise add to the manufacturing costs of such axial cages.